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ep4002 - gone wrong!

hi
i have an ep4002 controlling my c/heating[seperate boiler for h/w], it wont turn off at night despite checking all settings to be off by 9.00pm, i.e. the heating continues to run, rads hot, gas bill the size of a small planet. Ii have swapped out the controller for a spare - no luck......I have reduced the rad stats to low, droppped the room stat etc, could it be a by pass valve / zone valve [or something else] - not too clued up on these things.
Any help would be appreciated
pete

thanks for your help, i think you may be right. 'll try to get some wiring diagram from Baxi tomorrow.
Can the boiler still come on even if the display and light on the timer unit are showing as switched off?

It could be a faulty zone valve - most of the 2-port and 3-port zone valves used in domestic installs have an end-switch that normally closes its contact when the valve is opened. In most cases this end-switch is used to enable the boiler and make the pump run. The timer and room stat are wired in series to the valve motor - when the timeclock and stat call for heat then the valve is energised, opening the valve and activating the end-switch which in turn switches on the boiler and pump. If the valve is stuck open then this could be bringing on the boiler and pump even though the timer & stat are "off".